1. Technical Field
The present invention relates to an image processing technique, and more particularly to a technique for performing a super-resolution process.
2. Background Art
The importance of image processing is increasing as the camera-equipped mobile telephones and digital cameras become widespread. One of such image processes is a super-resolution process, also known as “digital zooming”. The super-resolution process is used for arbitrarily enlarging a captured image, and is important in the editing process performed after capturing images.
Various methods have been proposed for the super-resolution process. As ordinary methods, there are methods using interpolation such as, for example, the bilinear method (linear interpolation process) or the bicubic method (Non-Patent Document 1). With interpolation, however, the process can only produce intermediate values of sampled data. Therefore, when synthesizing an image enlarged by a factor of 2×2 or more, the sharpness of the edge, and the like, is likely to deteriorate, resulting in a blurred image. In view of this, there is proposed a method that uses an interpolated image as an initial enlarged image, and then extracts edge portions so as to enhance only the edges (Patent Document 1, Non-Patent Document 2). With this method, however, it is difficult to differentiate between an edge portion and noise, and the process is likely to also enhance noise as it enhances the edge portions, thus leading to a deterioration in the image quality.
In view of this, there is proposed a method that uses a database as a method for enlarging an image while suppressing the image quality deterioration. Specifically, the process captures a high-resolution image in advance by using a high-definition camera, or the like, and also obtains a low-resolution image of the same object under the same environment as the captured high-resolution image. The low-resolution image may be obtained by, for example, using another camera, capturing the high-resolution image with a zoom lens and then changing the zoom factor, and sub-sampling the high-definition image through a low-pass filter. Many such pairs of low-resolution images and high-resolution images are prepared, and the relationship therebetween is learned in a database as a super-resolution method. The super-resolution process is realized by using the database.
Such a method using a database does not require an enhancement process as described above, and is therefore capable of realizing a super-resolution with less image quality deterioration. For example, as such a process, a method in which an image is divided into blocks and the blocks of image are learned is known in the art (for example, Patent Document 2).
Patent Document 1: U.S. Pat. No. 5,717,789 (FIG. 5)
Patent Document 2: Japanese Patent No. 3278881
Non-Patent Document 1: Shinji Araya, “Meikai 3-Jigen Computer Graphics (3D Computer Graphics Elucidated)”, Kyoritsu Shuppan Co., Ltd., pp. 144-146, Sep. 25, 2003
Non-Patent Document 2: Makoto Nakashizuka, et al., “Image Resolution Enhancement On Multiscale Gradient Planes”, Journal of the Institute of Electronics, Information and Communication Engineers D-II Vol. J81-D-II, No. 10, pp. 2249-2258, October 1998